This invention relates generally to certain rare-earth oxyhalide phosphors activated with thulium for conversion of X-rays to visible light in multi-layer type X-ray screens exhibiting improved performance. More particularly, a specific concentration level for the thulium activator ion in said phosphors yields a better image in the associated photographic film without sacrificing either brightness or film speed.
In U.S. Pat. No. 3,795,814, assigned to the present assignee, there is disclosed certain lanthanum and gadolinium oxyhalide luminescent materials activated with thulium for use in X-ray image converter devices including X-ray screens. The thulium activator level in said phosphors is disclosed to be in the range from about 0.00025 to 0.01 moles per mole of the selected oxyhalide and is said to produce photographic images of improved brightness at a given level of X-ray exposure when employed in multi-layer type X-ray screens. Film speeds are also said to be improved with said phosphor material as compared with other phosphors in commercial use. There is further recognition of a more efficient conversion response by the phosphor attributable to its ultraviolet-blue emission characteristic. The optimum thulium activator concentration is said to reside between 0.002 and 0.003 moles thulium per mole of phosphor for the reported improvement in light production efficiency.
In more recently issued U.S. Pat. No. 4,070,583, also assigned to the present assignee, there is disclosed other thulium and terbium coactivated rare-earth oxyhalide phosphors also found useful in multi-layer type X-ray screens. Said phosphors are reported to exhibit increased ultraviolet emission when compared with the same rare-earth oxyhalide phosphor activated with terbium alone which is further recognized to help reduce undersirable light crossover between multiple emulsion layers in the associated photographic film. It is also further recognized in this same regard that incorporation of an ultraviolet-absorbing dye in the photographic film base can further reduce crossover by absorption of the ultraviolet radiation.
The present phosphors still further reduce crossover effects by shifting the ultraviolet emission produced when said phosphor is excited with X-rays. More particularly, certain rare-earth oxyhalide phosphors activated with thulium ion alone produce a higher proportion of lower wavelength radiation in the ultraviolet and near ultraviolet spectral region when the thulium ion concentration is maintained below a critical level in the previously discovered concentration range for said phosphor.